The invention relates to an electrode for medical applications, particularly an implantable stimulation electrode.
Electrodes for medical applications are employed in the form of effectors and sensors. What is meant by effectors are electrodes with which a stimulating effect is exerted. Sensors are electrodes used for measuring. Examples of effectors are stimulation electrodes for heart pacemakers as well as electrodes for the stimulation of nerves and muscles. Particularly coming into consideration as sensors are microelectrodes for potential pick-up as well as EEG and ECG electrodes, i.e. electrodes for sensing brain and heart currents, respectively.
In most cases, implantable stimulation or stimulating electrodes, for example for heart pacemakers, consist of an electrode shank having an insulated cable lead and of an electrode head for the transmission of the stimulation pulses, i.e. the active or effective area of the electrode. Essentially two demands are made of such electrodes:
1. The electrode material must be compatible with the body, i.e. the formation of connective tissue layers should be very low if it is not suppressed altogether; in any case, the thickness should remain below one hundred microns (100 .mu.m). The stimulation threshold, further, should remain largely constant.
2. A high double layer capacitance should form at the phase boundary electrode/bodily fluid, so that the polarization rise during the stimulation pulses (0.5 through 1 ms, 1 Hz, 10 mA, 10 mm.sup.2) remains less than 0.1 V.
The high double layer capacitance that is required has a beneficial effect in the case of stimulation electrodes and generally in the case of effectors as well because, as a result of the impressed current, only slight potential changes occur, electrochemical reactions with the bodily fluid are largely suppressed and the energy outlay is slight. In the case of sensors in which only a small measurement current flows, a high capacitance of the electrodes relaxes the demands that are made of the input impedance of amplifiers; noise is also reduced.
The demands cited above are particularly well met by electrodes wherein the electrode head, i.e. the active region in general, consists of glassy carbon. The high double layer capacitance of up to 0.1 F/cm.sup.2 (.nu.=1 Hz) is achieved by means of an activation of the surface of the glassy carbon, whereby a thin, firmly adhering layer of activated carbon is obtained, i.e. a surface with a microporous structure.
Activated glassy carbon thus repesents an electrode material having high capacitance that, beyond this, also exhibits good bodily compatibility and can therefore replace the metallic materials such as platinum/iridium for stimulation electrodes, platinum and tungsten for microelectrodes and silver/silver chloride for ECG electrodes which effect a degeneration of the adjacent tissue. Given glassy carbon, on the other hand, problems arise with respect to the mechanical processing (during manufacture) and the contacting, this in turn not being the case when metal electrodes are used.